1. Field of the Invention
This invention relates to a cam shaft holding system for holding cam shafts on a cylinder head in a multicylinder internal combustion engine wherein a plurality of valves consisting of at least one intake valve and at least one exhaust valve are positioned for each cylinder and opened and closed by cams secured to intake and exhaust cam shafts.
2. Description of the Prior Art
In internal combustion engines, in order to minimize size and improve performance, it is desired to minimize the included angle between intake valves and exhaust valves. This is because, when the included angle between the intake valves and the exhaust valves is small, it becomes possible to decrease the volume of the combustion chamber and, therefore, it becomes possible to increase the compression ratio. Also, it becomes possible to minimize the surface area of the combustion chamber. As a result, it becomes possible to improve the combustion efficiency.
In order to minimize the included angle between the intake valves and the exhaust valves, it is necessary to decrease the distance between an intake cam shaft for opening and closing the intake valves and an exhaust cam shaft for opening and closing the exhaust valves. However, when the distance between the intake cam shaft and the exhaust cam shaft is simply decreased, cam shaft holding members for holding the intake cam shaft and the exhaust cam shaft on the cylinder head become close to and interfere with cylinder head mounting bolts for mounting the cylinder head on a cylinder block.
In order to eliminate the above-mentioned problem, it has been proposed, for example in Japanese Utility Model Publication No. 53(1978)-643, to position one cam shaft bearing between the intake valve driving cams for each cylinder and another between the exhaust valve driving cams for each cylinder.
FIG. 3 is a plan view showing the conventional cam shaft holding system for a multivalve internal combustion engine. In this system, an intake cam shaft a and an exhaust cam shaft b are positioned above the upper surfaces of cylinder head mounting bolts c, c, . . . . . Between cams a1 and a2 of the intake cam shaft a and between cams b1 and b2 of the exhaust cam shaft b are positioned cam shaft bearings d, d for holding the intake cam shaft a and the exhaust cam shaft b on the cylinder head.
In the aforesaid cam shaft holding system, it is possible to prevent the cam shaft bearings d, d which hold the intake cam shaft aand the exhaust cam shaft b on the cylinder head from interfering with the cylinder head mounting bolts c, c, . . . . However, since the cam shaft bearings d, d are positioned in very narrow spaces between the adjacent intake cams a1 and a2 and between the adjacent exhaust cams b1 and b2, the widths of the cam shaft bearings d, d must be small. Therefore, the widths of the cam shaft holding portions of the cam shaft bearings d, d must also be small. This is disadvantageous from the viewpoint of engine performance, i.e. wear resistance of the cam shaft holding portions of the cam shaft bearings d, d, or the like.
Specifically, when the intake cam shaft a and the exhaust cam shaft b are rotated, a considerably high surface pressure or the like is exerted thereby on the cam shaft holding portions of the cam shaft bearings d, d. Therefore, in order to prevent the cam shaft holding portions of the cam shaft bearings d, d from breaking, it is necessary to adjust the spring constants, the spring member sizes and the spring coil diameters of the valve springs used in the intake valves and the exhaust valves which are operated by the intake cam shaft a and the exhaust cam shaft b. However, when the widths of the cam shaft holding portions of the cam shaft bearings d, d are small, the spring constants, the spring member sizes and the spring coil diameters of the valve springs must be decreased to prevent the cam shaft holding portions of the cam shaft bearings d, d from breaking. In this case, the valve springs become unable to follow high speed operation of the engine. Further, when the spring member sizes and the spring coil diameters of the valve springs are small, stresses arising in the members of the valve springs increase excessively, and it becomes necessary to replace the valve springs frequently.
Further, it has been proposed in Japanese Patent Publication No. 58(1983)-28416 to position a first cam shaft holding member and a second cam shaft holding member on a cylinder head and secure them to each other by use of bolts positioned on cylinder head mounting bolts coaxially therewith. In this system, through no problem arises with regard to the sizes of the cam shaft holding portions, the configuration of the cam shaft holding system becomes complicated. Also, when work concerning the cylinder head mounting bolts is conducted, it becomes necessary to remove other members in advance.